Tool wear propagation in Ti6Al4V laser assisted micro milling using micro ball end mill

Transcription

1 Tool wear propagation in Ti6Al4V laser assisted micro milling using micro ball end mill Zazuli Mohid *, E. A. Rahim Universiti Tun Hussein Onn Malaysia, Parit Raja, Batu Pahat, Johor, MALAYSIA. *Corresponding author: KEYWORD Laser assisted Ball end mill Micromilling Wear ABSTRACT This paper presents an experimental study of laser assisted micro milling of Ti6Al4V using micro ball end mill. Tool wear propagation was observed and compared between conventional and laser assisted micro milling to evaluate the effectiveness of laser assisted machining technique in Ti6Al4V micro machining. It is confirmed that laser assisted micro milling technique managed to improve the tool life even when using micro ball end mill tool. The tools in laser assisted micro milling served different tool wear mode compared to conventional micro milling. The maximum flank wear in laser assisted micro milling reduced for approximately 50 % at machining distance of 6000 mm when the laser pulse width is increased from 1 to 2 ms. 1.0 INTRODUCTION Titanium alloy is widely used in sports, aviation and biomedical industries. Even though titanium alloy is excellent in strength, fracture resistance and corrosion resistance, a lot of problems in fabrication process constrain its design and application. This alloy is categorised as notoriously difficult-to-machine material, where its machining is characterised by poor surface integrity and short tool life (Shokrani et al., 2016). A lot of techniques were introduced to improve the machining performance such as flood cooling, minimum quantity lubricant technique and cryogenic machining technique. It is also well reported that laser heating is capable in improving the tool life by cutting force reduction in various type of hard to machine materials (Ito et al., 2017; Pan et al., 2017; W.-S. Woo & Lee, 2015; W. S. Woo & Lee, 2018). However, it is important to understand that there are also some cases reported where the laser heating brought adverse effect to tool life when the cutting speed and heating parameters were not determined appropriately (Bermingham et al., 2015). Received 26 June 2018; received in revised form 3 August 2018; accepted 20 November To cite this article: Mohid and Rahim (2018). Tool wear propagation in Ti6Al4V laser assisted micro milling using micro ball end mill. Jurnal Tribologi 19, pp Malaysian Tribology Society (MYTRIBOS). All rights reserved.

2 Additionally, less has been discussed on laser assisted micro milling using micro ball end mill (Ayed et al., 2014; Xi et al., 2014). The machining process using micro ball end mills involves more complicated removal mechanism compared to micro flat end mill. The cutting speed in micro ball end mill process varies with the depth of cut which significantly gives effect to the machining performance such as cutting forces, surface roughness, machined surface profile and burr formation (Pratap & Patra, 2017). The radial shape and the existence of chisel in micro ball end mill are some of the main factors contribute to large rubbing and ploughing effect (Qui et al., 2014). The chemical affinity between tungsten carbide and Ti6al4V alloy leads to adhesive wear promoted by decarburization of tungsten carbide at the tool and workpiece interaction surface (Wang et al., 2014). On top of that, the effectiveness of laser assisted micro milling (LAµMill) can only be achieved when the thermal softening gives dominant influence during the machining process compared to ductility and strain hardening effect (Kumar et al., 2012; Singh & Melkote, 2007). The appropriate heating temperature is one of the issues being argued among researchers. The most appropriate temperature for titanium alloy Ti6Al4Vwere reported in different value which ranged from 150 to 500 C (Bermingham et al., 2012; Dandekar et al., 2010; Sun et al., 2011). However, in micro level of titanium alloy (Ti6Al4V) milling, small changes in heating temperature could give significant effect due to the workpiece ductility and low heat conductivity (Froes, 2015). Furthermore, even though it is proven that laser heating manages to reduce chipping wear mechanism (Sun et al., 2011), attrition, diffusion, and plastic deformation are remaining unavoidable (Jawaid et al., 2000). The purpose of this study is to evaluate the performance of laser assisted micro milling of Ti6Al4V using micro ball mill in term of tool wear propagation characteristics. The tool wear was measured and compared between conventional micro milling (Conv. µmill) and laser assisted micro milling (LAµMill) performed in dry condition. 2.0 EXPERIMENTAL PROCEDURE The experimental setup of laser assisted micro milling machine is shown in Figure 1. TiAlN coated cemented carbide micro ball end mill with diameter of 0.3 mm was used to perform linear groove machining. Air bearing micro spindle was set with angle of 80 from X-Y plane to reduce the effect of rubbing and ploughing mechanism. Linear grove machining of 25 mm/path was performed on Ti6Al4V with depth of cut (t c ) of mm. The experiment was done in two stages using different parameter as shown in and Error! Reference source not found.. The first stage (1 st. stage) was done to evaluate the influence of feed (f), cutting speed (v c ) and laser heating. The second stage (2 nd. Stage) was done to further evaluate the influence of laser heating at lower cutting speed (3.0 m/min) and different pulse repetition rate (t p =1 and 2 ms). In LAµMill, the laser beam was focused with an angle of 55 from X-Y plane at distance (X t-b ) of 0.6 mm. In the 1 st stage, the laser managed to heat the cutting area to approximately 128 C to 178 C while in the 2 nd stage, the laser managed to heat the material approximately 50 C higher that the 1 st stage (Mohid & Rahim, 2018). 89

3 Parameters Depth of cut, t c (mm) Feed, f 10-3 (mm/flute) Cutting speed, v c (m/min) Figure 1: Laser assisted micro milling setup. Table 1: Milling parameters. Value/ range , 3.0, st. stage: 7.6, nd. stage: 3.0 Table 2 : Laser heating parameters (applied only in LAµMill). Parameters Value/ range Laser focus diameter, (mm) Elliptical, Pulse repetition rate, f p (Hz) 100 Pulse width, t p (ms) 1 st. stage: 1 2 nd. stage: 1,2 3.0 RESULTS AND DISCUSSION In the 1 st. stage of machinability study, the maximum flank wear (VB MAX ) was measured after every 500 mm of linear cutting distance (X c ) and plotted in Error! Reference source not found.. The tool wear propagation was observed and measured repeatedly until mm of machining distance. In the case of Conv. µmill, at lower v c of 7.6 m/min and 10.6 m/min, increasing the f from 2.1 to mm/flute has consequently reduce the VB MAX for about 30 %. The uncut chip thickness increment is suggested effectively enhanced the cutting mechanisms by reducing the rubbing and ploughing effect. At f of mm/flute, the uncut chip thickness was too close to the cutting edge radius ( mm) and largely influence by size effect and formation of severe negative rake angle (Jackson et al., 2016). Even though it is reported that the minimum uncut chip thickness for Ti6Al4V is vary between 0.15 to 0.49 of the tool edge radii (Rezaei et al., 2018), the minus rake 90

4 angle effect could dominate the machining performance. The uncut chip thickness of micro ball mill is extremely small at the bottom side of the tool-workpiece contact point. However, at higher v c of 15.1 m/min, increasing the f from 2.1 to mm/flute has inversely increased the VB MAX for about 60 %. When the v c was 15.1 m/min and the f was increased to mm/flute, the cutting mechanism has started to have different adiabatic shearing characteristics due to different amount of heat accumulated at the shearing area (Molinari et al., 2002). As a result, the chips were formed in ununiformed shapes (Figure 3) which also indicates that the removal process were delivered inconsistently thus promoted the tool wear propagation. In LAµMill, different trend of VB MAX changes were observed when the f is increased. At v c of 7.6 m/min, the VB MAX was reduced for about 50 % when the f was increased from 2.1 to mm/flute the reduction rate was larger than Conv. µmill due to the softening effect. However, when the v c were 10.6 m/min and 15.1 m/min, another different VB MAX trend were observed. The VB MAX at f of 2.1 and mm/flute were approximately 100 % higher than when the f was mm/flute. Comparatively too small and too large f value will give adverse effect to the machining performance. The same trend was reported by Lee & Dornfeld, (2005). Even though that the LAµMill machining was performed in more consistent condition compared to Conv. µmill (Figure 4), low uncut chip thickness could cause drastic increment on the specific cutting pressure (Sooraj & Mathew, 2011). On the other hands, mm/flute of f is considered too large which consequently exposed the tool to severe adhesion and attrition risk due to cutting force increment (Lee & Dornfeld, 2005). Max. Flank Wear, VB MAX ( m) v c = 7.6 m/min f = 2.1 x 10-3 mm/flute f = 3.0 x 10-3 mm/flute f = 4.2 x 10-3 mm/flute v c = 10.6 m/min Conv. µmill LAµMill v c = 15.1 m/min Cutting Distance X c x 10 3 (mm) Figure2: Maximum flank wear increment by machining distance under different feed and machining methods and machining speed (v c ). 91

5 v c = 7.6 m/min v c = 10.6 m/min v c = 15.1 m/min mm/flute mm/flute 0.1mm Figure 3: Chip pattern at different feed and cutting speed in Conv. µmill (Mohid & Rahim, 2018). v c = 7.6 m/min v c = 10.6 m/min v c = 15.1 m/min mm/flute mm/flute 0.1mm Figure 4: Chip pattern at different feed and cutting speed in LAµMill (Mohid & Rahim, 2018). Figure 5 shows the conditions of the micro ball end mill tool after 50 mm of cutting length at different feed and machining techniques. Conv. µmill has shown different mode of tool wear mechanism compared to LAµMill. At f of mm/flute, Conv. µmill has shown no sign of tool damage, not even a small micro crack along the cutting edge. It indicates that the tool managed to stand the load during the machining process. However, workpiece debris were seeming to be easily adhere on the cutting tool near the chisel area. Increasing the f to mm/flute has sufficiently reduce the tendency of adhesion at the cutting tool edges. At f of mm/flute, the cutting tool edges were clear from damages or even any adhesion. The removal process was done under sufficient uncut chip thickness, thus produced loose arch chips as shown in Figure 3. Less adhesion mechanism and good chips flow characteristics are suggested to be contributed largely to the cutting edges shapes conservancy. 92

6 a f = mm/flute f = mm/flute Conv. µmill a b LAµMill Adhesion b Attrition Chipping Figure 5: Tool wear and adhesion in different machining conditions (v c = 7.6 m/min). When laser is applied to heat the workpiece in LAµMill, different tool conditions were observed. However, no sign of thermal crack was observed from the tool. The workpiece were heated to at most 178 C which is far lower than the allowable maximum temperature of 350 C reported by previous researchers (Bermingham et al., 2015). At f of mm/flute, a lot of adhesion were observed on the cutting tool flank surface. There are two factors suggested for the adhesion on the flank surface. Firstly, the material become softer and increased in elasticity and ductility. At a value of uncut chip thickness equal to cutting tool edge radius, the undeform chip thickness become unstable for the elastic recovery and rubbing effect (Luo et al., 2017). In this study, the chips were formed in ununiformed thickness and shapes. The thin and connected chips were compressed and rubbed in between the cutting tool and workpiece, and finally resulted to more obvious chip adhesion on flank surface when compared to Conv. µmill. Laser beam reflection and direct irradiation on the chips and debris is suggested to be the second factor contributed to flank surface adhesion (Error! Reference source not found.). The workpiece debris and chips which rotate along with the cutting tool were melted and adhered on the flank surface when they were directly interacted with the laser beam. When the f was increased to mm/flute, LAµMill has shown attrition and chipping wear at the upper side of the cutting tool edge. However, the wear was not located in the cutting tool working area or so called as tool effective diameter. This phenomenon was not identified in Conv. µmill. At larger f, larger chips were produced in LAµMill. Cutting temperature increment has made the chips produced in connected form as shown in Figure 4. As the removal process were performed repetitively, the chips flow to the upper side of the cutting tool and compiled in front of the cutting edges. Blocked by the compiled chips, the cutting edges bared extensive pulsing and concentrated force during the machining process. Chipping occurred on the coating layer located at the upper side of cutting edged due to the thick chip s adhesion. The wear propagated by attrition mechanism when the coating layer faded away and exposed the substrate surface to direct contact surface (Figure 5-b). It is hard to say that the wear shown in Figure 5 is a notch wear which reported in most machinability study (Bermingham et al., 2015; Bhopale & Pawade, 2014; Kasim et al., 2013). Even though the mechanism is similar, which is initiated by repetitive 93

7 load near the workpiece surface, the shape of notch was not obvious due to the tilted tool orientation applied. The tilted tool also provides proper cooling time to the tool thus decreases the tool wear rate (Pratap & Patra, 2017). The workpiece top surface and burr formed during machining were having contact with the cutting tool edged in comparatively large area instead of limited area. Even though the wear seems to be large, the value does not depict in Figure 2 because it located outside of the cutting tool effective area. The wear does not give effect to the machining surface quality. However, it could initiate fatigue failure when the size expands, and forms blunt cutting edge. When the coating material peeled off, diffusion wear accelerates thus exposes the tool to larger chipping and cutting-edge breakage (Wang et al., 2015). Further observation on VB MAX was done until mm of machining distance and found out that micro ball end mill performs better at f of mm/flute (Figure 2). The VB MAX decreases with the v c increment. The result has shown that f of mm/flute are the most appropriate value compared to 2.1 and mm/flute. At f of mm/flute, rubbing and ploughing effect increase due to softening effect initiated by cutting speed increment. At f of mm/flute, the uncut chip thickness was too big and does not have enough space to flow out from the machining area smoothly, thus increased the chips adhesion and compression mechanism problems. The smallest VB MAX at cutting distance of mm was observed from LAµMill performed under 15.1 m/min and mm/flute of v c and f, respectively. Even though the VB MAX at machining distance of mm was slightly larger than others, the VB MAX value did not show noticeable increment. It is suggested that the thermal softening initiated by laser heating, chips size and chip thickness were in the optimal condition for LAµMill of Ti6Al4V. Figure 6 shows the flank wear (VB MAX ) increment of the tools used in three different groove machining techniques using v c of 3.0 m/min in the 2 nd stage of machinability study. In Conv. µmill, no significant different of VB MAX between three different f values. However, machining using f of mm/flute has shown slightly lower wear rate compared to the others two f values. Even though the uncut chip thickness is the largest at f of mm/flute, the tool can sustain better compared to the machining using lower f value. At f of mm/flute, the chips were produced in conical shape and suggested to have better chip flow characteristics compared to the lower f value which produced connected chips. Even though theoretically, the force act on the cutting tool edges will increase when the f is increased since the shear, contact zone and sliding zone are increased (Groover, 2011), the force act on the cutting edges is still far lower than the maximum force the tool could take. The f increment of approximately mm interval was not large enough to initiate different tool wear characteristics. The flank wear was largely influenced by other factors than cutting forces. From the flank wear observation, it can be concluded that there is no linear relation between f and VB MAX can be seen from all machining techniques. However, it is proven that increasing the laser heating pulse width (t p ) from 1 ms to 2 ms has further improved the tool performance. The VB MAX reduced to half of VB MAX measured in Conv. µmill 94

8 Max. Flank Wear, VB MAX ( m) Max. Flank Wear, VB MAX MAX ( m) Max. Flank Wear, VB MAX ( m) f = f 2.1 = 2.1 x 10 x mm/flute f = f 3.0 = 3.0 x 10 x mm/flute f = f 4.2 = 4.2 x 10 x mm/flute Conv. Conv. µmill µmill LAµMill 1 1 LAµMill Cutting Cutting Distance, X c c Xx (mm) 10 X 3 (mm) X 3 c x 10 3 c x (mm) c 10x 3 (mm) 10 3 (mm) f = mm/flute Conv. µmill f = mm/flute LAµMill 2 Figure 6: Maximum flank wear increment by machining distance under different feed and machining methods. The mode of wear was also recorded differently in laser assisted milling when sufficient heating parameters were applied. Figure 6 shows two representative tool pictures captured from Conv. µmill and LAµMill performed at f of mm/flute. In Conv. µmill, the width of flank wear recorded the largest value at some distance away from the chisel area. The coating layer was ripped off from the substrate by repetitive attritions and adhesions. Thicker uncut chip thickness has made the wear propagated faster at a distance near to tool effective diameter. In contrast, LAµMil 2 has shown remarkable flank wear decrement at larger radial distance. It is an evidence showing that the removal process was performed under enough lase heating condition. However, at the chisel area, the wear condition was found similar in the both machining techniques. This area performs removal process at uncut chip thickness far smaller than the cutting tool edge radius. The effect of rubbing due to positive rake angle was too dominant, thus the softening effect could not give significant influence on the tool wear propagation rate and mode. Abrasive wear mechanism was found to be the major factor for wear propagation near the chisel area in the both Conv. µmill and LAµMill machining methods. 4.0 CONCLUSIONS From the experimental study under, the wear mechanism and performance of micro ball end mill in LAµMill was investigated. The main conclusions obtained from this study are as follows: (a) The wear at the cutting edges developed mainly by adhesion and attrition wear mechanisms. Chisel area served severe wear by abrasive wear mechanism in the both machining techniques, Conv. µmill and LAµMill. (b) Feed at mm/flute gives the lowest tool wear rate in LAµMill 1. Low and considerably equal tool wear rate were observed at all cutting speed. (c) Laser heating using pulse width of 1 ms in LAμMill 1 managed to reduce the tool wear rate but not at feed of mm/flute. Increasing the pulse width to 2 ms (LAμMill 2) has 95

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